1. Field of the Invention
Exemplary embodiments of the present invention relate to a method for reducing power consumption in a wireless communication system; and, more particularly, to a method capable of reducing power consumption of a station (STA) by quickly determining whether or not to receive a signal which is being received, through a service field existing at the front of a data region, in a wireless communication system.
2. Description of Related Art
In general, a wireless communication system such as a wireless LAN (WLAN) system includes a plurality of access points (AP) which are connected to one another through a network. A plurality of STAs existing within a basic service set (BSS) managed by one AP simultaneously transmit/receive frames to and from the AP. Meanwhile, an independent BSS (IBSS) includes a plurality of STAs, and one STA transmits/receives frames at the same time as the other STAs.
The frame transmission process in such a wireless communication system will be described as follows.
The AP transmits information which is required by a plurality of STAs to transmit sounding/feedback or receive data, through a control frame. When sounding/feedback is required, the STAs perform sounding/feedback to the AP. The AP transmits data by using the information from the STAs. The STAs receiving the data transmit information on whether or not an error occurred in the received data to the AP.
FIGS. 1A to 1C show the formats of protocol data units defined in the IEEE 802.11 related to the WLAN system. FIG. 1A shows the format of a PPDU (PLCP (Physical Layer Convergence Procedure) Protocol Data Unit) of the IEEE 802.11a/g for legacy STAs. FIG. 1B shows the format of a PPDU, which is defined in the IEEE 802.11a, in a HT (High Throughput) mixed environment in which legacy STAs and HT STAs are mixed. FIG. 1C shows the format of a PPDU, which is defined in the IEEE 802.11ac, in a VHT (Very High Throughput) mixed environment in which legacy STAs, HT STAs, and VHT STAs are mixed.
Referring to FIGS. 1A to 1C, the PPDU format for legacy STAs includes a legacy short training field L-STF, a legacy long training field L-LTF, a legacy signal field L-SIG, a service field, and a data field DATA.
Referring to FIGS. 1A to 1C, the PPDU format in the HT mixed environment in which legacy STAs and HT STAs are mixed includes legacy training fields L-STF and L-LTF, a legacy signal field L-SIG, a HT signal field HT-SIG, HT training fields HT-STF and HT-LTF, a service field, and a data field DATA.
Recently, in the IEEE 802.11 TGac, standardization has been discussed on a VHT wireless system capable of providing a maximum transmission rate of 1 Gbps at a MAC SAP (Service Access Point). In order to maintain frequency efficiency while satisfying such a high transmission rate, an AP and a STA should support a larger number of streams than four streams supported in the IEEE 802.11n. Therefore, a large number of antennas are required.
Considering the complexity or power consumption of the STA, it is difficult for the STA to support a large number of antennas. Accordingly, multi-user MIMO technology in which an AP transmits data to a plurality of STAs at the same time is being considered. In the VHT WLAN system, the multi-user MIMO technology is applied to consider the PPDU format as shown in FIG. 1C.
Referring to FIG. 1C, the PPDU format for VHT STAs includes legacy training fields L-STF and L-LTF, a legacy signal field L-SIG, a VHT signal field VHT-SIG-D which may be decoded by all STAs, VHT training fields VHT-STF and VHT-LTF, a VHT signal field VHT-SIG-D which may be decoded only by a VHT STA, a service field, and a data field DATA.
In the PPDU format for VHT STAs, information for demodulating the data field DATA is transmitted through the fields L-SIG, HT-SIG, VHT-SIG-C and VHT-SIG-D. Each of the fields L-SIG, HT-SIG, VHT-SIG-C and VHT-SIG-D has a limited size, because a preset number of bits are transmitted by using a predetermined modulation method and a channel code rate. Furthermore, the signal fields such as the fields L-SIG, HT-SIG, VHT-SIG-C and VHT-SIG-D do not contain information by which an STA can be identified. Accordingly, the MAC (Medium Access Control) address of a data region of a currently-received signal should be checked to recognize the destination of the signal.
In order to determine whether or not the MAC address is received without an error, a CRC (Cyclic Redundancy Check) bit positioned at the last of a MAC protocol data unit (MPDU) should be checked. Therefore, a STA should receive at least one MPDU to recognize the destination of a currently-received signal. Accordingly, even when the received signal is not a signal of which the destination is set to the STA, the STA should receive at least one MPDU.
In general, a data region containing a service field includes a service field, an MPDU field, and a padding field, when the number of MPDUs is set to one, and includes a service field, a plurality of delimiter fields, a plurality of MPDU fields, and a padding field, when the number of MPDUs is set to two or more.
When the number of MPDUs is set to one or more, a 16-bit service field is transmitted at a start point of the data region. In a transmitter, all the 16 bits of the service field are set to “0”, and then transmitted through a scrambler as shown in FIG. 2. In a receiver, a scramble initial value is estimated through the 16 bits of the service field, and the estimated scramble initial value is used for descramble.
In general, the scramble initial value is randomly given. For example, when it is assumed that the scramble initial value is “1011101”, a scramble sequence having a length of 127 bits becomes “0110110000011001101010011100111101101000010101011111010010100011011100001111111000111011110010110010010000001000100110001011101”.
When it is assumed that data input containing 16 “0” bits is “000000000000000000100000010000. . . ”, scrambled data out in FIG. 2 becomes “0110110000011001100010011000111 . . . ”. Therefore, the first 16 bits become a scramble sequence.
When it is assumed that an error does not exists in a signal received in the service field, the signal received in the service field becomes “0110110000011001” which is identical to the scramble sequence. Therefore, the signal received in the service field may be used to recognize the initial value of the scramble sequence.